The invention relates generally to clearance measurement systems, and more particularly to, a multi tip clearance measurement system for measuring a clearance between a stationary component and a movable component of a rotating machine.
Various types of sensors have been used to measure the distance between two objects. In addition, these sensors have been used in various applications. For example, a steam turbine has a rotating bucket that is disposed adjacent a carrier. The clearance between the rotating bucket and the carrier varies due to various operating conditions, such as changes in temperature, oxidation of the bucket tip, and so forth. It is desirable that a gap or clearance between the rotating bucket and the carrier be maintained during operation of the steam turbine.
One existing sensor is a multi tip capacitance sensor that includes multiple probe tips to measure capacitance for estimating the radial and axial clearance between stationary and movable components. Typically, the estimated clearance is a function of gain of all the signals from all the probe tips of the sensor. Unfortunately, an error in this gain, or a drift over a period time causes the system performance to deteriorate. Further, certain systems employ signal processing to evaluate the health of the individual signals from the probe tips and transfer functions are used to convert the measured signal to clearance. Again, error correction of individual gain of the probe tips is not addressed in such systems.
Moreover, these clearance measurement systems are typically employed to measure clearances between components during design and offline testing. Unfortunately, these existing systems are ineffective for in-service measurements due to the noise and drift generated by changes in the geometry of the components, among other factors. Instead, in-service clearance control is based on the clearance measurements previously taken during design and offline testing of components. As the components become worn during service, the offline measurements become ineffective for in-service clearance control.
Accordingly, a need exists for providing a multi tip clearance measurement system that provides an accurate measurement of clearance between two components by detecting and correcting single tip errors and drifts. It would also be advantageous to provide a self-calibrating clearance measurement system that could be employed for accurate clearance measurement for parts in operation.